The present invention relates in general to refiners for treating paper pulp fibers to condition the fibers prior to delivery to a papermaking machine and to refiners for handling stock having a consistency of about 3 to about 6 percent fiber by weight.
Disc refiners are used in the papermaking industry to prepare paper pulp fibers for the forming of paper on a papermaking machine.
Paper stock containing three to six percent dry weight fibers is fed between closely opposed rotating discs within the refiner. The refiner discs perform an abrading operation on the paper fibers as they transit radially between the opposed moving and non-moving refiner discs. The purpose of a disc refiner is to abrade the individual wood pulp fibers.
Processing of fibers in a low consistency refiner may be performed on both chemically and mechanically refined pulps and in particular may be used sequentially with a high consistency refiner to further process the fibers after they have been separated in the high consistency disk refiner.
In operation, a low consistency disc refiner is generally considered to exert a type of abrasive action upon individual fibers in the pulp mass so that the outermost layers of the individual cigar-shaped fibers are frayed. This fraying of the fibers, which is considered to increase the freeness of the fibers, facilitates the bonding of the fibers when they are made into paper.
Paper fibers are relatively slender, tube-like structural components made up of a number of concentric layers. Each of these layers (called "lamellae") consists of finer structural components (called "fibrils") which are helically wound and bound to one another to form the cylindrical lamellae. The lamellae are in turn bound to each other, thus forming a composite which, in accordance with the laws of mechanics, has distinct bending and torsional rigidity characteristics. A relatively hard outer sheath (called the "primary wall") encases the lamellae. The primary wall is often partially removed during the pulping process. Raw fibers are relatively stiff and have relatively low surface area when the primary wall is intact, and thus raw fibers exhibit poor bond formation, with the result that paper which is made of raw fibers has limited strength
It is generally accepted that it is the purpose of a pulp stock refiner, which is essentially a milling device, to partially remove the primary wall and break the bonds between the fibrils of the outer layers to yield a frayed surface, thereby increasing the surface area of the fiber multi-fold.
Disc refiners typically consist of a pattern of raised bars interspaced with grooves. Paper fibers contained in a water stock are caused to flow between opposed refiner discs or plates which are rotating with respect to each other. As the stock flows radially outwardly across the refiner plates, the fibers are forced to flow over the bars. The milling action is thought to take place between the closely spaced bars on opposed discs.
Disk refiners have proven to be cost effective devices with high throughput which can readily operate over a range of stock flows. Nevertheless, improvements in disk wear life and other means of reducing maintenance remain desirable.